Wrap fixed_arrays in a struct.

I did this to avoid issues that pop up when trying to take the address
of an array.

Something like this...

```c
int a[10];
p_a = &a;
```

... doesn't really mean what we want it to. But something like this...

```c
struct a { int data[10]; };
struct a an_a;
p_a = &an_a;
```

...does.